1. Field of the Invention
The invention relates to a structure for mounting the winding ends of a stator winding in a dynamoelectric machine (which is also referred to herein by other terms as a dynamo, a large electric machine or a turbo generator). The dynamo comprises a rotor and a stator. The stator comprises a sheet metal assembly with stator core end plates and a stator housing enveloping the sheet metal assembly. The rotor is mounted to the housing to rotate within said stator sheet metal assembly and stator core end plates. A stator winding is arranged in grooves in the sheet metal assembly facing said rotor. The ends of the stator winding project axially beyond the sheet metal assembly. The mounting structure comprises an inner ring, which may conform directly to the stator winding ends or a thin spacer may be interposed between the inner ring and the stator ends. The stator ends expand like a hollow cone. The mounting structure also comprises an outer support and means for clamping the stator ends between the inner ring and the outer support. The means for clamping are braced against the face of the sheet metal assembly and/or the stator housing.
2. Discussion of the Background
The invention is an improvement over state of the art mounting structures such as that disclosed, for example, in European patent document EP-A-O 432 720.
The attachment of stator winding ends (winding head) of turbo generators presents specific problems as a consequence of the high stresses to which these winding heads are subjected. The stresses are caused by the electromagnetic force effects, in particular during short-circuiting, oscillations, and different thermal expansions of the stator windings with respect to the stator iron (sheet metal core assembly and stator core end plates) during service. These stresses result sooner or later in loosenings and wear phenomena, which lead finally to line-to-ground fault and short-circuiting.
A series of systems have been introduced to improve the strength of winding heads of large electric machines, the majority of which operate with one-piece rings made of insulating material as the central support element.
German Offenlegungsschrift 1 613 125 discloses a structure for mounting the winding ends of a stator winding in an electric machine comprising an outer ring which conforms externally to a portion of the outer layers of the winding ends and an inner ring, which is independent of the outer ring. The inner ring engages the inner layers of the winding ends. To prevent the inner and outer rings from moving axially with respect to each other, C-shaped brackets envelop the winding ends and are secured to the rings. The two rings are secured together by the C-shaped brackets and are maintained axially spaced from the face of the stator sheet metal assembly by axially extending members secured to the outer ring at one end and to the face of the sheet metal assembly at the other end.
German Auslegeschrift 1 463 796 discloses an axially movable, radially fixed winding head mounting for the stator winding of a turbo generator. The winding heads are separated by means of wedges and spacers and held rigidly between two concentric insulating rings. In addition, the outer insulating ring is attached with respect to the housing sections in the radial direction by way of sliding wedges. The sliding wedges permit the outer insulating ring to move axially due to thermal expansion of the winding head. The outer insulating ring is connected to the stator housing by way of elastic elements.
The winding head mounting disclosed in U.S. Pat. No. 4,800,314 corresponds, on the whole, to that disclosed in German Auslegeschrift 1 463 796, but uses only one outer ring. The winding head ends are fastened by wedges in groups, cemented, and tied with ribbons to this outer ring.
The aforementioned embodiments of winding head supports have given largely satisfactory results in operational practice. However, they have the drawback that during inspection and in particular in the framework of retrofitting (replacement/repair of system critical parts for the purpose of increasing the life span and/or output) time-consuming disassembly and modifications have to be carried out. Thus, destruction of the outer one-piece support ring in order to even remove it is often unavoidable. Owing to the local peculiarities (i.e., cramped working space and expanding cone shape of the stator winding ends), it is then no longer possible to install a new one-piece outer ring in the framework of the retrofit.
To overcome the described problems, a support system is proposed in U.S. Pat. No. 4,563,607 that can be applied to both new constructions and in the framework of retrofitting. There, two adjacent bottom and upper winding ends are connected together in the winding head by means of clamping elements made of a synthetic resin-saturated glass cord.
Another solution to the support problem that is provided in particular for retrofitting is the subject matter of U.S. Pat. No. 5,140,740 or its corresponding European patent document EP-A-0 432 720. The mounting proposed there comprises an inner ring which conforms to the stator winding ends. The stator winding ends expand outwardly like a hollow cone. The outer support comprises an at least two-piece ring or support beams which are connected together and extend in the circumferential direction. Tie bolts, which engage with the inner ring and the outer ring or the support beams, serve to clamp the inner ring and the outer support. As an alternative, the outer ring and the tie bolts can be dispensed with as in the prior art (FIG. 4). Then the inner ring and the front brackets (i.e., stator winding ends) of the winding head are braced by interposition of clamping wedges which bear against angular members. The angular members are attached to the face of the stator sheet metal assembly.
The drawback with the first alternative of this construction is that it requires greater complexity on the connecting side (non-driven side) of the machine. In the alternative without the outer ring or without the support beam, unacceptably high tensile stresses can occur at the face of the stator sheet metal assembly.